This invention concerns dusting device such as used in coal mines, and particularly concerns such a device which can more effectively provide low levels of dust, e.g., limestone dust, of uniform densities or solids concentrations to the mine ventilation air currents for transportation thereby over substantial distances.
In underground coal mines, large amounts of coal dust are necessarily generated by the cutting, blasting, loading, crushing and the like of the coal. This dust not only is present in high concentrations at these sites but is carried by the mine ventilation air current from these sites, as well as from belt conveyor transfer point and locations along the belt conveyor where ventilation checks are installed, throughout the mines until it settles out on down stream surfaces. Such atmospheric dust, and settled-out dust accumulations are fire and explosion hazards and must be periodically cleaned up or made inert by addition of limestone rock dust.
There are several ways in which underground coal mines generate coal dust, (1) when the coal is cut from the coal seam, (2) when the coal is loaded into equipment that takes the coal to a feeder, (3) when the feeder crushes the coal, (4) when the feeder discharges the coal onto a conveyor belt and, (5) when the coal is discharged from one conveyor belt to the next conveyor belt.
This coal dust is carried by the ventilating air stream into the return ventilation air entries where it settles out in these entries and along the conveyor belt. The dust is also carried along the belt conveyor entries by the air stream in these entries.
Federal law requires that intake entries be 65% inert, and that return entries be 80% inert. Limestone dust is spread in the intake and return entries to get the needed percentages of inertness.
Federal law also does not allow coal dust to remain on top of surfaces underground. For example, an entry could have the percent inertness required but if coal dust is on top of the limestone dust and the entry is black with coal dust, a violation of federal law could be cited.
Prior Art: To keep the return entries and belt conveyor entries white, a limestone distributor called a Trickle Duster is typically employed. These dusters have an air blower and a tank that holds limestone dust. The blower discharge air goes into the tank typically up thru the limestone dust and fluidizes the dust. A small portion of the limestone dust is carried by the air stream and is continuously discharged into the entries. The trickle duster discharges a continuous fog of limestone dust that is carried by the ventilation air and helps to keep the return entries and conveyor belt entries white. A serious problem with conventional such dusters arises from the blowing of ambient air into the limestone dust tank. Anytime air is taken underground and is warmer than the underground ambient temperature (normally 50-55° F.), a condition of 100% humidity is present. In summer months, all surfaces in an underground coal mine are wet. The air that is blown into the limestone dust is at 100% humidity and the limestone dust absorbs water during its fluidization which causes particles of the dust to become heavier and to stick together. When this limestone dust is discharged into the air stream entries, it will not travel as far and will not uniformly coat surfaces within the mine because it is wet and particles of the dust have agglomerated.
Another serious problem arises from the custom that trickle dusters are powered either by an electrical motor or by a rubber lagged roller which is in contact with and driven by the conveyor belt. Trickle dusters receiving power from a roller in contact with the conveyor belt have the problems of the lagged roller shaft breaking and of excessive lagging wear. This lagged roller typically is as long as the belt conveyor is wide. Therefore, a 60″ wide belt conveyor will have a trickle duster lagged roller about 60″ long at the lagging, and the supporting shaft of the lagged roller will be even longer so that end bearings can be installed.
A belt conveyor has the highest tension in the center of the belt along its longitudinal axis such that when the lagged roller is forced against the belt conveyor, the load on the lagged roller is concentrated at the center of the roller's length. This causes the roller to flex thus causing its center shaft to break and, among other problems, cause the lagging on the roller to wear rapidly in the center of its length.